It is well known in the art that in NPN transistors where polycrystalline silicon (polysilicon) is used as the emitter contact to a monocrystalline silicon substrate, a very thin film is formed at the interface between the polysilicon and the silicon substrate, usually in the form of a contaminated thin oxide layer, which layer can critically affect the operation of the transistor and impact its current gain. Such structures are shown, for example, in U.S. Pat. Nos. 4,483,726 to Isaac et al., 4,431,460 to Barson et al., and 4,467,519 to Glang et al., each of which is assigned to the assignee of the present invention. It is therefore important to detect the presence and thickness of this oxide film so as to predict and/or control the performance of the transistor.
In order to measure the presence of this oxide film and thereby obtain a correlation between the current gain of the transistor and the thickness of the interfacial film, Secondary Mass Spectroscopy (SIMS) analyses are routinely performed. In a SIMS analysis, the sample is sputtered away with an ion-gun and the departing materials are analyzed in a mass spectrometer. The lateral area, otherwise called sputtering front, can be as large as 100 microns.sup.2. The thickness of the film, on the other hand, can be so thin that its presence is measured at approximately 2 to 3.times.10.sup.15 atoms/cm.sup.2, and determined only by suitably integrating the area under the SIMS curve that is associated with the sputtering of the oxygen.
Measuring an interfacial oxide layer, whether by determining its thickness or by finding the oxygen concentration, is an important requirement in the early stages of a processing cycle to avoid manufacturing a transistor that does not incorporate an appropriate interfacial film. Currently, the SIMS analysis is routinely performed on a wafer immediately after deposition of the emitter polysilicon, usually on a control wafer.
A method for measuring electrical potentials in solid state matter, and more particularly, when the solid state matter is hidden beneath at least one conductor and one insulator layer, has been described in U.S. Pat. No. 4,609,867 to Helmut Schink. In Schink, an electron beam induced current related to a potential drop across an oxide layer is determined. Carriers are excited within the oxide layer allowing the excitation volume to create a conductive path, and measurement of this current can be used to determine the potential. This technique is effective for thick oxide layers on the order of approximately 1000 A, but is inadequate for measurements of the thickness of the oxide film at a silicon/polysilicon interface, which is usually on the order of a few Angstroms.
A similar approach is described in U.S. Pat. No. 4,296,372 to Hans-Peter Feuerbaum, wherein a voltage is applied to a conductive layer through the insulating material which extends over a conductive region to be tested. A voltage measurement taken at that region provides a good indicator of the electrical integrity of the tested region underneath. The same disadvantage found in Schink's teaching applies to this teaching, where no information regarding the thickness of the insulating film is provided.
Measurement of surface photovoltages with back surface illumination and without making ohmic contact has been advantageously used in silicon-on-saphire (SOS) structures, providing a non-destructive testing of SOS wafers. This technique is described in U.S. Pat. No. 4,859,939 to Jonathan I. Gittelman, et al. This method uses a light source to provide radiant energy to the device under test. Gittelman, et al., however, does not show or suggest its use for measuring the thickness of oxide films in polysilicon/silicon structures. Rather, Gittelman et al. obtains a measurement of the quality of the semiconductor film by determining the lifetime of the carriers that are generated.
In summary, no non-destructive system is known which is sufficiently sensitive to determine the thickness of a polysilicon/silicon interfacial oxide film in the Angstrom range.